Electronic system for high power load control

ABSTRACT

An electronic control system for a high power load is provided with a plurality of parallel current paths divided into two groups (11-12), with control devices (Q 1  -Q n ) in the current paths of one group each having a current limiting resistor (13), and control devices (Q 21 , Q 22 ) in the current paths of the other group each having no current limiting resistor, so that when the control devices of the second group are turned fully on, a short circuit is achieved by the arrangement of parallel current paths. Separate but coordinated control signals (1, 2) are provided by suitable means (P 1  -P 3 ) to first turn on the control devices of the first group and increase their conduction toward saturation as a function of control input, and when fully on, or shortly before, to turn on the control devices of the second group and increase their conduction toward saturation as a function of the control input as that input continues to increase. Electronic means (32-35) may be employed to generate control signals (1, 2). The system may be used for I-V characteristic measurements of solar arrays, as well as for other load control purposes.

ORIGIN OF INVENTION

The invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of Section 305 of theNational Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat.435; 42 USC 2457).

BACKGROUND OF THE INVENTION

This invention relates to an electronic system which controls currentfrom a high voltage, high power source in response to a low inputsignal, and thereby produces as an output an amplification of the inputsignal.

Many electronic devices are available for control of current to a loadin response to a low input signal, such as an electron tube ortransistor. Most have the ability to produce an output at all timesproportionate to its input over a substantial range. Transistors are, ofcourse, the preferred devices because of their low power losses.However, their use to control high voltage, high power systems has beenlimited. For example, when such an electronic control system isconnected to a solar array for testing, such as to plot itscurrent-voltage (I-V) characteristic curve, the open circuit voltage(V_(oc)) may be over 300 volts, and the short circuit current (I_(sc))may be over two amperes. This is well over the rated maximum of mostcommercially available electronic load control systems.

Although high voltage power transistors are readily available, problemsof second breakdown (in bipolar transistors) and power dissipation limitany single transistor to operation at relatively low currents when athigh voltage or low voltages when at high current. Thus, a singletransistor may be satisfactory for switching between these two states,but cannot carry a sustained high current at high voltages. Use of highvoltage bipolar transistors to control the load requires relativelylarge numbers of parallel current paths, and great care in matchingthese currents, to prevent current hogging and second breakdowndestruction of the transistors.

Power MOSFET's with high voltage ratings and reduced on-resistance havenow become available. These transistors are immune to second breakdown,have larger safe operating areas of their current-voltage characteristiccurves and are easy to operate in parallel. However, to stay withintheir safe operating region, it is still necessary to use multipleparallel current paths or current limiting resistors in series with theMOSFET's, except at lower voltages. These resistors would prevent thecontinuation of the I-V curve down to zero voltage because of thevoltage drop across them. The present invention provides an arrangementof parallel current paths which overcomes this deficiency, permittingthese parallel current paths to share the current load from open circuit(V_(oc)) to short circuit (V_(sc)).

SUMMARY OF THE INVENTION

In accordance with the present invention, a plurality of parallelcurrent paths are divided into two groups, with control devices in thecurrent paths of one group each having a current limiting resistor, andcontrol devices in the current paths of the other group each having nocurrent limiting resistor so that when the control devices of the secondgroup are turned fully on, a short circuit is achieved by thearrangement of parallel current paths. Separate but coordinated controlsignals are provided by suitable means to first turn on the controldevices of the first group and increase their conduction towardsaturation as a function of control input, and when fully on, or shortlybefore, to turn on the control devices of the second group and increasetheir conduction toward saturation as a function of the control input asthat input continues to increase. When ganged potentiometers (P₁, P₂)are used to produce the signals (1 and 2), a third potentiometer (P₃) isused to provide a fixed offset for one signal (1) chosen to causedevices controlled by it to reach saturation at the time, or shortlyafter, the second signal (2) turns the second group of devices on.Electronic means (32-35) may be used to generate the control signals (1,2), each with the same slope as a function of a control input signal.The system may be used for I-V characteristic measurements of solararrays, as well as for other load control purposes.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims. The inventionwill best be understood from the following description when read inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of an exemplary embodiment of the invention.

FIG. 2 illustrates a graph constant voltage and constant current regionsof an I-V characteristic curve for a solar array, and the safe operatingarea A of power transistors.

FIG. 3 is a graph illustrating two coordinated control signals generatedfor the system of FIG. 1 as a function of a control input.

FIG. 4 is a block diagram of the system of FIG. 1 in use for plotting anI-V characteristic curve of a solar array.

FIG. 5 is a schematic diagram illustrating the use of the presentinvention as an amplifier to drive a load from a high power source witha small input signal.

FIG. 6 is a graph illustrating coordinated control signals for thesystem of FIG. 5 as a function of a small input signal.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1 of the drawings, a preferred embodiment of theinvention is disclosed for an exemplary application, namely a portable,manual sweep, high-voltage electronic load to perform I-V curvemeasurements on a solar array 10 where open circuit voltages may be ashigh as 350 volts, and short circuit currents as high as 2.5 amperes.However, this exemplar is only by way of illustration, and not by way oflimitation. It will be readily appreciated that the present inventionmay be used for other purposes, such as an amplifier for a servomotor ora large audio amplifier, as will be more explicitly pointed outhereinafter.

It should also be recognized that although transistors Q₁ -Q_(n) andQ₂₁, Q₂₂ are used in this exempler, other electronic control devices maybe used, such as electron tubes. Power transistors have a safe operatingarea of the I-V realm, bordered by a more-or-less parabolic curve ofmaximum power dissipation indicated by an area A below a parabolic curveshown in FIG. 2. Also shown in FIG. 2 is an idealized solar array I-Vcurve, consisting of two quite different regions, a constant voltageregion B and a constant current region C.

High power MOSFET's ("Hexfets" from International Rectifier Corporation)were chosen for the transistors instead of the commonly used bipolartransistors for two reasons. First, they are immune from secondbreakdown and have a relatively high safe operating region on the I-Vplot, limited only by the package power dissipation rating. Second,their inherently self-limiting current characteristic (with temperature)is an advantage. This makes it rather simple to operate several MOSFET'sin parallel, with no problems of current mismatch or "hogging".

At lower voltages, the array 10 acts somewhat as a constant currentsource, while at high voltages (approaching the open circuit voltage)the array behaves as a nearly constant voltage source. As indicated inFIG. 2, a single transistor might be capable of handling the entirearray current near the short circuit condition (low voltage) but couldhandle only a fraction of the array current, shown as I max (H_(v)),near the high voltage limit of the array.

For moderate voltage arrays (e.g., 50 volts), a group of transistorsoperating in parallel can split the array current to solve this problem.However, as the open-circuit voltage is increased, there is acorresponding decrease in I max (H_(v)), necessitating the use ofproportionally more parallel transistors with attendant problems ofcomplexity, cost and matching.

An alternative approach is to use a series resistance in eachtransistor-controlled current path. This reduces the voltage on thetransistor as the current increases and can thus keep the transistorwithin its safe operating region. However, the series resistances ineach parallel current path would constitute a minimum load, preventingthe array voltage from reaching the short circuit current condition (0volts).

To overcome these problems, two groups of parallel current paths (11,12) are used, each with a MOSFET device for electronic control ofcurrent therethrough. Each of the transistors Q₁ through Q_(n) of thefirst group 11 includes a separate series (current limiting) resistor,such as resistor 13 connected to the drain of the transistor Q₁. Thisfirst group of transistors, operating in parallel covers the highvoltage range extending from the open circuit condition (V_(oc)) in theconstant voltage region B part of the way into the constant currentregion C. The second group of transistors Q₂₁, Q₂₂ are turned on whenthe first group have reached the fully-on state. They then take over theload and approach the short circuit current (I_(sc)) condition.

The control signals for the two groups of transistors operated inparallel are applied to their control (gate) electrodes over leads 1 and2 from ganged potentiometers P₁ and P₂ connected to a source of DC biasvoltage 14. A separate isolating resistor R is connected in series withthe gate of each device. As the moving contact (wiping arm) of thepotentiometers move in unison from the bottom towards the top in FIG. 1,the signal on line 2 increases linearly from zero to +DC, while thesignal on line 1 increases from a predetermined offset voltage V_(A) to+DC. The signal on line 1 crosses the threshold level, V_(TH), forturning on the transistors first, as shown in FIG. 3, so only the firstgroup of transistors conduct. When the signal on line 1 reaches thesaturation control level, V_(SAT), the first group of transistors arefully on. At the same time, the signal on the line 2 reaches thethreshold level for turning on the transistors, at which time the secondgroup of transistors are turned on. At that time, or shortly before, thesecond group of transistors without current limiting resistors begin toconduct. They take over the task of increasing the load on the solararray as they are turned on and approach the short circuit condition atsaturation.

Since both control signals reach +DC at the same time, it is apparentthat the rate at which the second group of transistors are turned on isgreater than for the first group of transistors. There is thus linearityonly as to each group. For the two groups taken together, there is achange in the slope of the control signal from one group to the next.This is not a problem for this application of I-V curve measurements forthe solar array 10. For other applications, this could be corrected byplacing a resistor of appropriate value in series between thepotentiometer P₂ and the DC source 14, in which case the signal on line2 would not reach the +DC level, but would provide a signal that variesas a function of the degree of rotation of the potentiometer (i.e., thecontrol input) with the same slope as the signal 1. Some adjustment inthe offset voltage V_(A) and the DC source 14, would then be required toplace the intersection of the signal 1 with the saturation leveldirectly above the intersection of the signal 2 with the threshold levelfor the second group of transistors to be turned on as the first groupreaches saturation. Alternatively the two signals on lines 1 and 2 couldbe generated as shown in FIG. 5 and discussed below.

For this application of making I-V curve measurements, the current ismeasured as a voltage across a low value resistor 15 is parallel withtwo high value resistive branches, one of which includes a potentiometer16 for a digital voltmeter 17, and another which includes apotentiometer 18. The voltage is measured across a potentiometer in aresistive branch in parallel with a second resistive branch thatincludes a potentiometer 20 connected to a digital voltmeter 21. Thesedigital voltmeters permit reading out the current and voltage to a highdegree of accuracy. The potentiometers 18 and 19 permit scaled currentand voltage measurement signals to be derived for driving conventionalapparatus for plotting current versus voltage as the potentiometers P₁and P₂ are operated together from their lower position to their upperposition.

A diode 22 protects the transistors from high negative switchingtransients as a pair of ganged switches S₁ and S₂ are operated fromtheir normal position shown to their alternative (short) position, atwhich time the current for the short condition can be measured. A switchS₃ is provided for connecting a DC offset voltage source 23 in serieswhen array voltages of 2 volts or less is required, i.e., when plottingcurrent for very low voltages near zero volts. This offset voltagepermits plotting the I-V curve to zero volts and across the Y-axis to anegative voltage. Otherwise zero volts can only be approached, and notreached due to voltage drops across the small internal series resistanceof the transistors.

FIG. 4 shows the necessary connections of the electronic system shown inFIG. 1 to the array 10 and an X-Y plotter or recorder 24 and the DCvoltage source 23. The negative input of the system is floating, butcommon with X-Y negative outputs. This negative input may be connectedto a ground terminal 25 on the system chassis. This option permits I-Vmeasurements on a solar array which requires a floating ground;otherwise the negative input of the system is connected to the groundterminal.

For I-V characteristic measurements of the solar array 10, the controlpotentiometers P₁ and P₂ are initially turned fully down. With the solararray still disconnected, the digital voltmeters 17 and 21 should readzero current and voltage. With the switch S₃ in its alternate position,the voltmeter 21 should read near the voltage of the source 23. With theswitches S₁ and S₂ in their normal position, the array is connected tothe system. The voltmeter 21 should read the array open circuit voltage,V_(oc), and the ampere meter 17 should read zero. While the plotter 24is rendered operative, the I-V control potentiometers P₁ and P₂ are thenmoved upwardly to plot the I-V characteristic curve from V_(oc) to zerovolts.

In an exemplary embodiment, eight "Hexfet" transistors are included inthe first group and two in the second group. Each of the currentlimiting resistors in the first group is a 500 ohm, 50 watt seriesresistor, and each of the isolating resistors R in both groups is 150ohms. The maximum current which can be handled by the system is limitedby the current limiting resistors to 2.5 amperes. The 500 ohm value waschosen to keep the transistors within their safe operating region. Thetwo transistors without current limiting resistors are held off untilthe first eight are turned on to saturation, or very near saturation, bythe potentiometer P₁. Both potentiometers P₁ and P₂ are selected to be1K ohm. Each potentiometer operates over a different voltage range toset the different turn-on points for the two groups of transistors. Thepotentiometer P₃ is 100 ohms.

With current limiting resistors of higher wattage, fewer transistorswould be needed in the first group, or higher currents could be handled.For example, if it is desired to handle four amperes, the eighttransistors could be used with 125 watt current limiting resistor for0.5 ampere through each 500 ohm resistor. Alternatively, ten transistorsin the first group with 80 watt current limiting resistors wouldsuffice. In either case, four additional transistors without seriesresistors would be required to handle the high current, low voltageregion of the I-V curve, i.e., the "constant current region."

For use of the system as a high power amplifier, the elements 15 through18 would be replaced by a load 30, such as a servometer, and theswitches S₁ through S₃ and the elements 19 through 23 would be omitted,as shown in FIG. 5. In place of the DC source 14 and potentiometers P₁through P₃, an electronic control signal generator would be provided.

The electronic control signal generator responds to an input signal atterminal 31 and produces a linear output signal through two invertingoperational amplifiers 32 and 33 in cascade for the second group oftransistors 12. The input signal is also applied to two invertingoperational amplifiers 34 and 35 in cascade. An offset voltage (-V_(A))is applied to the amplifier 34. The gain of the two sets of cascadedamplifiers is equal to produce signals 1 and 2 as shown in FIG. 6.

It should now be apparent that a high power amplifier is provided by thesystem disclosed, which draws current from a high power source inresponse to a low current input signal. Although particular embodimentsof the invention have been described and illustrated herein, it isrecognized that modifications and variations may readily occur to thoseskilled in the art. Consequently, it is intended that the claims beinterpreted to cover such modifications and equivalents.

What is claimed is:
 1. A system for control of a plurality of electronic devices in parallel current paths to share a load current from open circuit or high voltage range to short circuit for high current range, with at least one device in each current path, each device having a control terminal, said system comprisinga connection to the control terminal of each device of one group for electronic signal control of said one group of devices in parallel, and a current limiting resistor in series with each device in said one group, a connection to the control terminal of each device of the remaining devices in another group for electronic signal control of said other group of devices in parallel, and no current limiting resistor in series with each device in said other group, means for generating a first signal in proportion to a control input for turning on said devices of said one group and increasing their conduction to saturation as said control input increases, and means for generating a second signal in proportion to said control input for turning on said devices of said other group when said devices of said one group reach saturation and increasing their conduction to saturation as said control input increases.
 2. A system as defined in claim 1 wherein said devices are MOSFET devices, each with an isolation resistor in series with its control terminal.
 3. A system as defined in claim 1 or 2 wherein said means for generating said first signal is comprised of a first potentiometer connected to a source of DC bias voltage and said means for generating said second signal is comprised of a second potentiometer connected to said source of DC bias voltage, said first and second potentiometers being ganged together for operation in unison, and means in series with said first potentiometer to offset the signal generated thereby with respect to the signal generated by said second potentiometer sufficiently to cause said first and second groups of devices to be turned on in sequence in response to a control input in the form of operation of said potentiometers.
 4. A system as defined in claim 1 or 2 wherein said means for generating said first signal is comprised of a first electronic amplifying means responsive to a control input in the form of an electrical signal for producing said first signal proportional thereto, said first electronic amplifying means including means for offsetting said control input signal amplified thereby, and a second electronic amplifying means responsive to said control input in the form of an electrical signal for producing said second signal proportional thereto, where said amplifying means each have the same gain factor, and said offset is selected for causing said first and second groups of devices to be turned on in sequence.
 5. A system as defined in claim 1 or 2 wherein said plurality of parallel current paths are connected to a solar array, and said devices are operated to measure the current versus voltage characteristics of said array with a high resistance voltage dividing network connected directly in parallel with said array for voltage measurements, and a low resistance current sensing means in series with said array for current measurements.
 6. A system as defined in claim 5 including switching means for shunting said parallel current paths for a short circuit current measurement.
 7. A system as defined in claim 5, including switching means for connecting a fixed source of offset voltage in series with said two groups of parallel current paths.
 8. A system as defined claim 1 or 2 wherein said two groups of parallel current paths are connected parallel to a source of power through a load. 